Circuit substrate, circuit-formed suspension substrate, and production methods therefor

ABSTRACT

The present invention provides a circuit substrate and a circuit-formed suspension substrate comprising the circuit substrate, the circuit substrate comprising a metal foil substrate and an insulating layer composed of a polyimide resin formed on the metal foil substrate, wherein the polyimide resin is one obtained by the reaction of (A) p-phenylene diamine and (B) acid anhydrides of (a) 3,4,3′,4′-biphenyltetracarboxylic acid dianhydride and (b) 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane. Since a coefficient of linear thermal expansion of the polyimide resin is close to that of various metal foils, the circuit substrate causes no cracking on the resin layer and scarcely causes warpage, and the resin layer does not separate.

FIELD OF THE INVENTION

The present invention relates to a circuit substrate, a circuit-formedsuspension substrate, and production methods thereof. More specifically,the present invention relates to a circuit substrate having aninsulating layer composed of a polyimide resin on a metal foilsubstrate, a circuit-formed suspension substrate having formed on thecircuit substrate a pattern circuit composed of a conductive layer, andproduction methods of the circuit substrate and circuit-formedsuspension substrate.

In a magnetic disk device such as a hard disk device which is used as anexternal storage device for a computer, etc., for performing magneticrecording or regeneration, it is necessary to relatively run theabove-described magnetic disk and a magnetic head, elastically push themagnetic head to the magnetic disk against to an air stream formedthereby, and keep a definite fine interval between the magnetic head andthe magnetic disk. A magnetic head supporting means for elasticallypushing the magnetic head to the magnetic disk against the air stream asdescribed above is a suspension. The present invention relates to acircuit substrate which can be suitably used for producing such acircuit-formed suspension device, a circuit-formed suspension substratehaving formed on such a circuit substrate a circuit composed of aconductive layer by a patterning technique, and the production methodsof them.

BACKGROUND OF THE INVENTION

Recently, a circuit substrate comprising a metal foil having formedthereon an insulating layer composed of a polyimide resin has been usedas a thin-film multilayer circuit substrate for the purposes ofhigh-density packaging of semiconductors and a high-speed signaltreatment. However, since the polyimide resin which is conventionallyused as an insulating material has a large coefficient of linear thermalexpansion, the circuit substrate is liable to cause cracking on theresin layer and warping and the delamination of the resin layer, andwarping.

On the other hand, in computers and storage devices which are peripheralequipments for computers, in addition to the improvement of thecapacity, small-sizing and a low cost have been required and with suchrequirements for a background, in particular, the technique of hard diskdrives has been greatly advanced. Also, in the magnetic head, recentlythe developments of a thin-film magnetic head (TFH) wherein the coilportion is made of a thin film and further a thin film-magneticresistance composite head (MR) which serves reading and writing and hasa greatly large storage capacity have been hurried to a conventionalmetal in gap (MIG).

However, by a conventional technique of constituting desired wiring bylaying a conductive wire on a suspension substrate, the conductive wiregives influences on the elastic modulus of the suspension to cause thefluctuation of the pushing force of the suspension and as the case maybe, by the contact with the magnetic disk, the durability of themagnetic disk device is sometimes lowered.

Therefore, recently, a suspension formed by directly forming an electriccircuit on a suspension substrate for mounting a magnetic head has beenpractically used. However, the suspension finally obtained using theconventional circuit substrate having a polyimide resin as an insulatinglayer also causes insulation failure and warping occur, resulting inperformance failure in some cases, since the polyimide resin has acoefficient of linear thermal expansion larger than that of a metal foilsubstrate.

SUMMARY OF THE INVENTION

The present invention has been made to overcome the above-describedproblems in the conventional circuit substrate comprising a metal foilhaving formed thereon an insulating layer composed of a polyimide resinand in the circuit-formed suspension substrate prepared by using thecircuit substrate.

Accordingly, one object of the present invention is to provide a circuitsubstrate wherein a coefficient of linear thermal expansion of thepolyimide resin is close to that of various metal foils, and hencecracking does not occur on the resin layer, the resin layer does notseparate, and warping scarcely occur.

Another object of the present invention is to provide a circuit-formedsuspension substrate using the circuit substrate.

Other objects of the present invention are to provide the productionmethods of the circuit substrate and circuit-formed suspension substrate

A circuit substrate comprises a metal foil substrate and an insulatinglayer composed of a polyimide resin formed on the metal foil substrate,wherein the polyimide resin is a polyimide resin obtained by thereaction of

(A) p-phenylene diamine and

(B) acid anhydrides of

(a) 3,4,3′,4′-biphenyltetracarboxylic acid dianhydride and

(b) 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane.

More specifically, the polyimide resin is one obtained by reacting apolyamic acid obtained by the reaction of

(A) p-phenylene diamine and

(B) acid anhydrides of

(a) 3,4,3′,4′-biphenyltetracarboxylic acid dianhydride and

(b) 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane in the presence of aphotosensitive agent.

The method of producing a circuit substrate according to the presentinvention comprises forming a coating film composed of a photosensitivepolyimide resin precursor on a metal foil substrate, light-exposing,heating after the light-exposure, developing, imidating the precursor byheating to form a circuit substrate having an insulating layer composedof a polyimide resin, wherein the photosensitive polyimide resinprecursor is obtained by compounding a polyamic acid obtained by thereaction of

(A) p-phenylene diamine and

(B) acid anhydrides of

(a) 3,4,3′,4′-biphenyltetracarboxylic acid dianhydride and

(b) 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane with a photosensitiveagent.

According to the method described above, by light-exposing the coatingfilm composed of the photosensitive polyimide resin precursor and formedon the metal foil substrate in conformity with a definite pattern,heating the film after light-exposure, developing the film to form thedefinite pattern composed of the above-described precursor, and finallyheating the pattern-formed film at high temperature to form polyimide,the circuit substrate having the patterned insulating layer composed ofthe polyimide resin can be obtained.

The circuit-formed suspension substrate of the present inventioncomprises a metal foil substrate, an insulating layer composed of apolyimide resin and formed on the metal foil substrate, and a patterncircuit composed of a conductive layer and formed on the insulatinglayer, wherein the polyimide resin is a polyimide resin obtained by thereaction of p-phenylene diamine and acid anhydrides of3,4,3′,4′-biphenyltetracarboxylic acid dianhydride and2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane.

Also, the method of producing a circuit-formed suspension substrateaccording to the present invention comprises forming a pattern circuitcomposed of a conductor layer on the insulating layer composed of thepolyimide resin of the circuit substrate described above, wherein thephotosensitive polyimide resin precursor for forming the insulatinglayer of the circuit substrate is obtained by compounding the polyamicacid obtained by the reaction of p-phenylene diamine and an acidanhydride of 3,4,3′,4′-biphenyltetracarboxylic acid dianhydride and2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane with a photosensitiveagent.

The circuit-formed suspension substrate is obtained by preparing theabove-described circuit substrate having the definite-patternedinsulating layer composed of the polyimide resin and then forming apattern-circuit composed of a conductive layer on the insulating layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a slant view showing an example of the circuit-formedsuspension substrate of the present invention,

FIG. 2 is a cross-sectional view taken along line A—A of thecircuit-formed suspension substrate shown in FIG. 1,

FIG. 3 is a cross-sectional view taken along line B—B of thecircuit-formed suspension substrate shown in FIG. 1,

FIG. 4, FIG. 5, FIG. 6, FIG. 7, FIG. 8, FIG. 9, FIG. 10, FIG. 11, FIG.12, FIG. 13, FIG. 14, FIG. 15, and FIG. 16 are sectional views of mainportions for showing the production steps of the circuit-formedsuspension substrate of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described in detail below.

According to the present invention, in the circuit substrate comprisinga metal foil substrate and an insulating layer composed of a polyimideresin and formed on the metal foil substrate, the polyimide resin is apolyimide resin obtained by the reaction of

(A) p-phenylene diamine

(B) acid anhydrides of

(a) 3,4,3′,4′-biphenyltetracarboxylic acid dianhydride

(b) 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane

In the present invention, there is no particular restriction on themetal foil substrate, but usually a stainless steel foil, a copper foil,an aluminum foil, a copper-beryllium foil, a phosphor bronze foil, a 42alloy foil, etc., are used. Furthermore, according to the presentinvention, a long metal foil is preferably used as such a metal foilsubstrate. That is, each insulating layer composed of the polyimideresin is formed on the long metal foil substrate in such a manner ofrepeatedly applying a definite pattern, each desired pattern circuitcomposed of the conductive layer is formed on each patterned insulatinglayer, and finally, the long metal foil substrate is cut every eachpattern circuit to provide each pattern-formed suspension substrate.

Thus, according to the present invention, the circuit substrate can beobtained by coating a solution of the photosensitive polyimide resinprecursor on a long metal foil substrate such as a long stainless steelfoil followed by drying to form a coating film, exposing the coatingfilm to ultraviolet rays via a mask having a definite pattern, heating(heating after light exposure), developing, and then applyingheat-curing (polyimidation of the polyamic acid) to form a definitepattern composed of the polyimide resin as the insulating layer. In theabsence of a photosensitive agent, components (A) and (B) can be reactedwith heating to form a polyamic acid or polyimide. The patterning of thecoating film composed of the polyimide can be effected in the continuoussteps, i.e., exposure, development, etching, etc. using a conventionalphotoresist.

The above-described photosensitive polyimide resin precursor is a liquidcomposition obtained by reacting the aromatic diamines and thetetracarboxylic acid dianhydrides described above at a substantiallyequimolar ratio in an appropriate organic solvent such asN-methyl-2-pyrrolidone, N,N-dimethyl acetamide, etc., at roomtemperature or, if desired, under heating, to form polyamic acid, andcompounding the polyamic acid with a photosensitive agent.

The proportion of 3,4,3′,4′-biphenyltetracarboxylic acid dianhydride ascomponent (a) is preferably from 70 to 99 mol %, more preferably from 80to 95 mol %, the proportion of2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane as component (b) ispreferably from 30 to 1 mol %, more preferably from 20 to 5 mol %. Byusing such kinds of the acid anhydrides in such amounts and p-phenylene,a polyimide resin having a low coefficient of linear thermal expansioncan be obtained. The coefficient of linear thermal expansion is in therange of 10 to 20 ppm/° C.

Taking the molar numbers of components (a) and (b) as a and b,respectively, the proportions (mol %) of components (a) and (b) as theacid anhydrides are (a/(a+b))×100 and (b/(a+b))×100, respectively.

As the photosensitive agent, the dihydropyridine derivative representedby formula (I):

wherein, X¹, X², X³, and X⁴ each independently represent a hydrogenatom, a fluorine atom, a nitro group, a methoxy group, an amino group, adialkylamino group (the alkyl moiety preferably having 1 to 4 carbonatoms), a cyano group, or a fluorinated alkyl group (the alkyl moietypreferably having 1 to 4 carbon atoms); Y¹ represents a cyano group or agroup represented by formula —COR³, Y² represents a cyano group or agroup represented by formula —COR⁴ (wherein R³ and R⁴ each independentlyrepresent an alkyl or alkoxy group having from 1 to 4 carbon atoms, ananilino group, a toluidino group, a benzyloxy group, an amino group, ora dialkylamino group in which the alkyl moiety preferably has 1 to 4carbon atoms); R¹, R², and R⁵ each independently represent a hydrogenatom or an alkyl group having from 1 to 3 carbon atoms; and at least oneof the combination of R¹ and R³ and the combination of R² and R⁴ may becombined to form a 5-membered ring, a 6-membered ring, or a heterocyclicgroup each containing a keto group, as described in detail inJP-A-6-75376 (the term “JP-A” as used herein means an “unexaminedpublished Japanese patent application”) can be used.

Examples of the photosensitive agent include4-o-nitrophenyl-3,5-dimethoxycarbonyl-2,6-dimethyl-1,4-dihydropyridine(hereinafter referred to as “niphedipin”),4-o-nitrophenyl-3,5-dimethoxycarbonyl-2,6-dimethyl-1-methyl-4-hydropyridine(hereinafter referred to as “N-methyl compound”), and4-o-nitrophenyl-3,5-diacetyl-1,4-dihydropyridine (hereinafter referredto as “acetyl compound”). Those can be used alone or as mixturesthereof. If necessary, an appropriate amount of imidazole is used as adissolution aid to a developer.

In the present invention, the dihydropyridine derivative described aboveis usually used in the range of from 0.05 to 0.5 molar part to 1 molarpart of the total amount of p-phenylene diamine and acid anhydrides.Imidazole which is used, if necessary, is usually used in the range offrom 0.05 to 0.5 molar part to 1 molar part of the total amount of thediamine component described later (including p-phenylenediamine) and theacid dianhydrides.

By coating a solution of the photosensitive polyimide resin precursor onan appropriate substrate, light-exposing (ultraviolet irradiation) thecoating film by irradiating the film with ultraviolet rays, and heating(heating after light-exposure), positive-type or negative-type latentimages are formed. By developing the latent images, positive-type ornegative-type images, that is, the desired patterns, are obtained. Thus,by finally heating the patterns at high temperature to imidate thepolyamic acid, a patterned coating film composed of the polyimide resincan be obtained.

In more detail, in the case of using the photosensitive imide resinprecursor described above, when the heating temperature afterlight-exposure is as relatively low as about 140° C., the exposedportions are dissolved in a developer to form a positive-type image,while when the heating temperature after light-exposure is as relativehigh as not lower than about 170° C., the non-exposed portions aredissolved in a developer to form a negative image, although theabove-described condition may differ a little according to the kind ofthe photosensitive agent used.

In this case, as the developer described above, an aqueous solution ofan organic alkali such as tetramethylammonium hydroxide, etc., or anaqueous solution of an inorganic alkali such as sodium hydroxide,potassium hydroxide, etc., is used. The alkali concentration is usuallyin the range of from 2 to 5% by weight. To the aqueous alkali solutionmay be added a lower aliphatic alcohol such as methanol, ethanol,n-propanol, iso-propanol, etc. The amount of the alcohol added isusually 50% by weight or lower. Also, the developing temperature isusually in the range of from 25 to 50° C.

In the case of using the photosensitive polyimide resin precursor foruse in the present invention, when the heating temperature afterlight-exposure is as relatively low as about 140° C., in the non-exposedportions, a hydrogen bond is formed between the imino group hydrogen atthe 1-position of the dihydropyridine derivative and the carboxy groupof the polyamic acid to decrease the hydrophilic property of thepolyimide resin and the diffusion speed of the coated layer into thedeveloper, which results in lowering the dissolution speed of the coatedlayer into the developer, while in the exposed portions, thedihydropyridine derivative which is a neutral compound is changed to abasic pyridine compound by the light-exposure, and the basic pyridinecompound forms polyamic acid and a weak salt structure, which results inincreasing the hydrophilic property of the coated layer to increase thedissolution speed of the coated layer into the developer. As describedabove, since, in this case, the dissolution speed of the exposedportions in the developer is larger than that of the non-exposedportions, the coating film after light-exposure, heating afterlight-exposure, and developing gives a positive-type image.

On the other hand, in the case of using the photosensitive polyimideresin precursor used in the present invention, when the heatingtemperature after light-exposure is as relatively high as not lower than170° C., in the non-exposed portions, the dissolution speed of thecoated layer in the developer is lowered as the case that the heatingtemperature after light-exposure is as relatively low as about 140° C.,while in the exposed portions, the dihydropyridine derivative describedabove is changed to a basic pyridine compound by the light-exposure, thebasic pyridine compound accelerates the polyimidation of polyamic acidto lower the dissolution speed of the coating film in the developer andat the same time, the pyridine compound itself is further changed to aninsoluble cyclic compound by heating after light-exposure as well asaccelerates the polyimidation of polyamic acid, which results in morelowering the solubility of the coating film at the exposed portions inthe developer as compared with the non-exposed portions. As describedabove, when the heating temperature after light-exposure is asrelatively high as not lower than about 170° C., since the dissolutionspeed of the exposed portions in the developer is greatly lower thanthat of the non-exposed portions, the coating film after light-exposure,heating after light-exposure, and developing gives a negative image.

According to the present invention, by coating a solution of thephotosensitive polyimide resin precursor as described above on a longmetal foil substrate followed by heat-drying to form a coating film ofthe precursor, light-exposing the coating film to a definite pattern byirradiating with ultraviolet rays through a mask, heating the film afterlight-exposure, developing to form, preferably, a negative image havingthe definite pattern, and then heat-curing the negative image to causean imidation reaction to form an insulating layer composed of thepolyimide resin, the circuit substrate of the present invention can beobtained. For forming the polyimide resin by heating the polyimide resinprecursor as described above, it is preferred to heat the coating filmthereof for several hours at a temperature of from about 350 to 400° C.in vacuo or in an inert gas atmosphere.

Thereafter, by forming each definite circuit composed of a conductivelayer having a definite pattern on the insulating layer according to aconventional method and also forming necessary terminals, and thenchemically cutting the long metal foil substrate into a desired form,the circuit-formed suspension substrate of the present invention can beobtained.

In the present invention, for improving the adhesive property of thepolyimide resin obtained to the substrate, if necessary, an aminogroup-containing bifunctional polysiloxane may be used as a part of thediamine components. Examples of such an amino group-containingbifunctional polysiloxane are polysiloxanes represented by followingformula (II);

wherein R¹ represents an alkylene group having from 1 to 18 carbonatoms; R² represents an alkyl group having from 1 to 18 carbon atoms;and n represents an integer of from 1 to 100.

The amino group-containing bifunctional polysiloxane is used in therange of 10 mol % or less of the diamine component composed ofp-phenylene diamine and the amino group-containing bifunctionalpolysiloxane.

Taking the molar numbers of p-phenylene diamine and the aminogroup-containing bifunctional polysiloxane as a and b, respectively, theproportion of the amino group-containing bifunctional polysiloxane (mol%) is (b/(a+b))×100.

In particular, in the formula (II) described above, R¹ is preferably analkylene group having from 1 to 7 carbon atoms and examples thereof aremethylene, ethylene, propylene, butylene, and hexylene. R² is preferablyan alkyl group having from 1 to 7 carbon atoms, and examples thereof aremethyl, ethyl, propyl, butyl, and hexyl. Also, n is preferably aninteger of from 1 to 40. Bis(aminopropyl)tetramethyldisiloxane ispreferably used as the amino group-containing bifunctional polysiloxane.

The circuit-formed suspension substrate of the present invention can beobtained as follows. After producing the circuit substrate as describedabove, by forming each pattern circuit composed of a conductive layer onthe insulating layer composed of the polyimide resin according to aconventional method of a patterning technique and also forming necessaryterminals, and then chemically cutting the insulating layer into adesired form, each circuit-formed suspension substrate can be obtained.

Since the circuit substrate has as an insulating layer a polyimide resinlayer obtained by the reaction of p-phenylene diamine and the specificproportion of the two kinds of acid anhydrides and the polyimide resinhas a coefficient of linear thermal expansion close to that of a metalfoil, cracking does not occur on the resin layer, the resin layer doesnot separate, and warping scarcely occur. Accordingly, thecircuit-formed suspension substrate obtained by providing a patterncircuit composed of a conductive layer on the circuit substrate does notcause performance failure such as insulation failure.

Furthermore, according to the present invention, since a precisepatterning work can be applied at a high sensitivity and a highcontrast, capacity increasing and small-sizing of the circuit-formedsuspension substrate are possible.

The circuit-formed suspension substrate of the present invention and theproduction thereof are explained in detail by referring to theaccompanying drawings.

FIG. 1 is a slant view showing an example of a circuit-formed suspensionsubstrate 1 of the present invention. As shown in FIG. 1, thecircuit-formed suspension substrate 1 has an insulating layer (notshown) composed of the polyimide resin on a stainless steel foilsubstrate 2 and a definite pattern circuit composed of a conductivelayer 3 is formed thereon as a thin layer. At the tip portion of thesubstrate is formed a gimbal 4 in a body by forming cuts in thesubstrate and a slider (not shown) having a magnetic head is fixed ontoit. At both end portions of the substrate are formed necessary terminals5 and 6, respectively. FIG. 1 shows the state that a cover layer forprotecting the surface of the substrate is released.

FIG. 2 is a cross-sectional view taken along line A—A in FIG. 1. Thecircuit-formed suspension substrate has an insulating layer 7 composedof the polyimide resin on a stainless steel foil substrate 2 and adefinite pattern circuit composed of a copper conductive layer 3 isformed thereon as a thin layer via a chromium thin layer 23. Theconductive layer is protected with a coating composed of a nickel thinlayer 28 and further terminals 5 each are formed thereon. The wholesurface excluding the terminals are protected with a coating film 8.

FIG. 3 is a cross-sectional view taken along line B—B in FIG. 1. Thecircuit-formed suspension substrate has an insulating layer 7 composedof the polyimide resin on the stainless steel foil substrate 2 and adefinite pattern circuit composed of the copper conductor layer 3 isformed thereon via a chromium thin layer 23. The conductive layer isprotected with a coating composed of a nickel thin layer 28 and furtherprotected by a coating film 8.

As the long stainless steel foil substrate, the substrate having thethickness in the range of usually from 10 to 60 μm, and preferably from15 to 30 μm, from the view point of the vibration characteristics, andthe width in the range of from 50 to 500 mm, and preferably from 125 to300 mm, is used. However, the long stainless steel foil substrate usedin the present invention is not limited to them.

FIG. 4 and FIG. 5 show an embodiment of the production steps of thecircuit substrate of the present invention, and FIG. 6 to FIG. 16 showan embodiment of the production steps of the circuit-formed suspensionsubstrate of the present invention.

First, as shown in FIG. 4, a solution of the photosensitive polyimideresin precursor is coated on the entire surface of the stainless steelfoil substrate as described above such that the thickness of the resinlayer obtained is from 2 to 20 μm, and preferably from 5 to 10 μm, andthe coating layer is heated at a temperature of from 60 to 150° C., andpreferably from 80 to 120° C., to form a coating 21 of thephotosensitive polyimide resin precursor.

The coating of the photosensitive polyimide resin precursor isirradiated with ultraviolet rays via a appropriate mask, whereby thecoating is light-exposed to an definite pattern. In this case, theintegrated light-exposure amount is in the range of usually from 100 to1,000 mJ/cm², and preferably from 200 to 700 mJ/cm². The light-exposurewavelength is in the range of usually from 300 to 450 nm, and preferablyfrom 350 to 420 nm. After the light exposure, the coating is heated(heating after light-exposure) at a temperature of from 80 to 200° C.,and preferably from 120 to 180° C. for from about 2 to 10 minutes, andthen subjected to a developing treatment. In the present invention, itis preferred to obtain negative images. The patterned coating of thepolyimide resin precursor thus obtained is heated at high temperature toform a polyimide resin, whereby a patterned insulating layer 22 composedof the polyimide resin is formed on the stainless steel foil substrate 2as shown in FIG. 5 to provide the circuit substrate of the presentinvention.

As shown in FIG. 6, a chromium thin layer 23 and a copper thin layer 24are continuously and successively formed on the entire surface of thestainless steel foil substrate 2 having the patterned insulating layer22 of the polyimide resin by sputtering. The existence of the chromiumthin layer 22 is useful for closely adhering the copper thin layer 24onto the insulating layer 22 composed of the polyimide resin. In thiscase, the thickness of the chromium thin layer is in the range ofpreferably from 100 to 600 Å, and the thickness of the copper thin layeris in the range of preferably from 500 to 2,000 Å. Also, the surfaceresistance of the copper thin layer thus obtained is usually 0.6 Ω/□ orless.

Thereafter, as shown in FIG. 7, electrolytic copper plating having athickness of from about 2 to 15 μm is applied to the copper thin layer24 to form a conductive layer 25 composed of copper. In FIG. 7, thechromium thin layer is not shown.

Then, as shown in FIG. 8 and FIG. 9, after carrying out a light-exposureand a developing treatment by a patterning technique using a liquidphotoresist 26 or a dry film laminate, the copper conductive layer 25 atthe non-pattern portions are removed by etching, whereby a definiteconductive pattern 27 composed of the copper is formed on the insulatinglayer 22 composed of the polyimide resin. In this case, etching of thecopper thin layer is preferably carried out by alkali etching.

After removing by etching the copper conductive layer of the non-patternportions, the chromium thin layer 23 is further removed by etching and adefinite conductive pattern 27 is obtained on the insulating layer 22composed of the polyimide resin as shown in Table 10. For etching thechromium thin layer 23, for example, a potassium ferrycyanide etchingsolution, a potassium permanganate etching solution, or a sodiummetasilicate etching solution is used.

After removing the unnecessary chromium thin layer on the substrate,electroless nickel plating is applied to form a hard nickel thin layer28 on the surface of the copper conductive layer 27 and the surface ofthe stainless steel thin substrate 2, whereby the surface of the copperconductive layer is protected. Accordingly, the thickness of nickelplating may be in such a degree that the copper conductive layer as thelower layer does not expose, and is in the range of usually from 0.05 to0.1 μm.

Thereafter, the conductive pattern 27 of the wiring portions areprotected by coating using the photosensitive polyimide resin precursor,and also terminal is formed at each of the necessary terminal-formingportions. Namely, the surface (excluding the terminal) is protected bycoating to form a cover layer. In FIG. 12 to FIG. 16, the left side ofthe substrate shows the formation of the wiring portion and the rightside thereof shows the formation of the terminal portion.

That is, as shown in FIG. 12, in the wiring portion, the conductivepattern 27 is coated with the polyimide resin 29. In theterminal-forming portion, excluding the terminal and a lead portion 30for forming the terminal by electrolytic plating by means of patterning,coating with the foregoing photosensitive polyimide resin precursor,light-exposure, heating after light-exposure, development, andheat-curing (imidation) are carried out in the same manner as above tocoat with the polyimide resin, whereby a coated layer 31 is formed.

As shown in FIG. 13, in the terminal-forming portion, the electrolessnickel plated thin layer 28 (see FIG. 11) which protects the surface ofthe conductor pattern 27 is released and at the same time, theelectroless nickel plated thin layer 28 on the stainless steel foilsubstrate 2 is also released. After coating the stainless steel foilsubstrate, the conductor pattern 27, and the polyimide resin coatingfilm 31 (except for the terminal-forming portion) with a photoresist bya method of using a photoresist according to a conventional method,electrolytic nickel plating 32 and electrolytic gold plating aresuccessively applied to the terminal-forming portion to form a terminal34. In this case, the thickness of electrolytic nickel plating and thethickness of electrolytic gold plating both are usually from about 1 to5 μm. Thereafter, the photoresist is removed.

As shown in FIG. 14, in the conductor pattern 27 having formed thereonthe terminal 34, the lead portion 30 (see FIG. 12) which is used forelectrolytic -plating is removed by chemical etching. The removal ofcopper and chromium at the lead portion may be performed in the samemethod as described above.

After removing the lead portion, for cutting the stainless steel foilsubstrate 2 by chemical etching, by carrying out a light exposure and adevelopment using a photoresist 35 or a dry film laminate according to aconventional method, a desired pattern is formed on the stainless steelfoil substrate 2 as shown in FIG. 15. The stainless steel foil substrateis then cut into a desired form by etching. In this case, as the etchingsolution, an aqueous solution of ferric chloride, cupric chloride, etc.,is used.

After the etching treatment, by washing with pure water and drying, thecircuit-formed suspension substrate 1 of the present invention can beobtained. That is, the circuit-formed suspension substrate has theinsulating layer 22 composed of the polyimide resin on the stainlessstain foil substrate 2 and has the conductor pattern 27 composed of thethin layer of the conductive layer on the insulating layer, that is, thepattern circuit, and the entire surface thereof (except for the terminal34) is protected by coating with the coating layer 31 composed of thepolyimide resin.

Since the circuit substrate has as an insulating layer a polyimide resinlayer obtained by the reaction of p-phenylene diamine and the specificproportion of the two kinds of acid anhydrides and the polyimide resinhas-a coefficient of linear thermal expansion close to that of a metalfoil, cracking does not occur on the resin layer, the resin layer doesnot separate, and warping scarcely occur. Accordingly, in thecircuit-formed suspension substrate, cracking does not occur on theresin layer, separation of the resin layer does not occur, and nowarping occurs, thereby causing no defects in performance.

EXAMPLES

The present invention is described in more detail by referring to thefollowing examples, but it should be understood that the invention isnot construed as being limited thereto.

Example 1

In 19.72 kg of dimethylacetamide were dissolved 0.702 kg (6.5 mols) ofp-phenylenediamine, 1.624 kg (5.5 mols) of3,4,3′,4′-biphenyltetracarboxylic acid dianhyride (diphthalic aciddianhydride), 0.444 kg (1.0 mols) of2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane (6FDA) (the sum of theacid anhydrides: 6.5 mols), and the resulting solution was stirred atroom temperature for 72 hours. The temperature of the solution wasraised to 75° C. When the viscosity of the solution reached 5,000 c.p.,heating was stopped, and the solution was allowed to stand to cool it toroom temperature.

By adding 0.9633 kg (2.78 mols) of niphedipin, 0.6422 kg (2.04 mols) ofan acetyl compound, and 0.161 kg (2.36 mols) of imidazole to thesolution, a solution of a photosensitive polyimide resin precursor wasprepared.

After coating the solution of the photosensitive polyimide resinprecursor on a long stainless steel (SUS 304) foil having a thickness of25 μm by a continuous coater, the coating layer was dried by heating at120° C. for 2 minutes to form the coating film of the photosensitivepolyimide resin precursor. The coating film was irradiated byultraviolet rays via a mask at a light-exposure amount of 700 mJ/cm² andafter heating the film at 160° C. for 3 minutes, the coating film wassubjected to a developing treatment to form negative-type images. Byfurther heating the coating film having the negative-type images at 400°C. in a vacuum of 0.01 torr, a patterned insulating layer (thickness 6μm) composed of the polyimide resin was formed to obtain a circuitsubstrate.

The coefficient of linear thermal expansion of the polyimide resin filmwas 17.0 ppm/° C. Also, in the case of a general SUS 304 stainless steelfoil, the coefficient of linear thermal expansion was 17 ppm/° C. Inthis case, the coefficient of linear thermal expansion was measuredabout a sample having a width of 2 mm, a thickness of 6 μm, and a lengthof 30 mm by a TMA method (thermal mechanical analysis method) at atemperature-raising speed of 10° C./minute and a load of 5 g.

A chromium thin layer having a thickness of 500 Å and a copper thinlayer having a thickness of 1,000 Å were formed on the insulating layercomposed of the polyimide resin of the circuit substrate by continuoussputtering treatments. The surface resistance of the copper thin layerwas from 0.3 to 0.4 Ω/□.

After adhering a lightly pressure-sensitive adhesive sheet to the backsurface of the stainless steel substrate as a mask, sulfuric acidelectrolytic plating was applied to the entire surface of the copperthin layer to form a conductor layer composed of copper plating having athickness of 10 μm.

After laminating a commercially available dry film laminate on theconductor layer at 110° C. according to an ordinary method, the laminatelayer was light-exposed at a light exposure amount of 80 mJ/cm², anddeveloped. The non-patterned portions of the copper conductive layerwere removed by alkali etching, whereby the conductive layer waspatterned such that the wiring portion, the terminal-forming portion,and the lead portion formed by electrolytic plating remained. The resistwas then removed. The stainless steel foil thus treated was immersed inan aqueous solution of a mixture of potassium ferricyanide and sodiumhydroxide at 25° C. to remove the unnecessary chromium thin layer.

An ordinary electroless nickel plating was applied to the stainlesssteel substrate to form a nickel thin layer having a thickness of about0.5 μm on the entire surface of the stainless steel foil including theconductive layer and the insulating layer.

As described above, a desired coating film was formed on the wiringportion and the terminal-forming portion of the conductive layer on thestainless steel foil using the photosensitive polyimide resin precursorin the same manner as above.

The substrate was immersed in a solution of a nitric acid releasingagent at room temperature to remove the electroless plated thin layer onthe terminal-forming portion and the stainless steel foil.

After coating with an ordinary photoresist, except for theterminal-forming portion, according to a conventional method,electrolytic nickel plating and electrolytic gold plating weresuccessively applied to the foregoing terminal-forming portion to formplating layers each having a thickness of 1 μm, whereby a terminal wasformed. The resist was then released.

After the plating treatments, for removing the lead portion used forplating from the conductive layer, copper alkali etching and chromiumetching were carried out in the same manner as above.

After removing the lead portion for plating from the conductive layer,for cutting the stainless steel foil substrate into a desired form,after forming each necessary pattern on the stainless steel foil bycarrying out a light exposure and a development using a photoresist or adry film laminate according to a conventional method, the stainlesssteel foil substrate was immersed in an etching solution of ferricchloride at 45° C. to cut the substrate into each desired form. Aftersufficiently washing with pure water, the cut substrate was dried toobtain each cut circuit-formed suspension substrate.

In the circuit-formed suspension substrate thus obtained, since thepolyimide resin has a low coefficient of linear thermal expansion,cracking does not occur on the resin layer, delamination of the resinlayer from the substrate does not occur, and the circuit-formedsuspension substrate has a high reliability and does not cause warping,thereby being free from performance failure.

Example 2

In 19.72 kg of N-methyl-2-pyrrolidone were dissolved 0.702 kg (6.5 mols)of p-phenylenediamine, 1.743 kg (5.9 mols) of3,4,3′,4′-biphenyltetracarboxylic acid dianhyride, 0.289 kg (0.65 mols)of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane (the sum of the acidanhydrides: 6.5 mols), and the resulting solution was stirred at roomtemperature for 72 hours. The temperature of the solution was raised to75° C. When the viscosity of the solution reached 5,000 c.p., heatingwas stopped, and the solution was allowed to stand to cool it to roomtemperature.

By adding 0.9633 kg (2.78 mols) of niphedipin, 0.6422 kg (2.04 mols) ofan acetyl compound, and 0.161 kg (2.36 mols) of imidazole to thesolution, a solution of a photosensitive polyimide resin precursor wasprepared.

After coating the solution of the photosensitive polyimide resinprecursor on a long stainless steel (SUS 304) foil having a thickness of25 μm by a continuous coater, the coating layer was dried by heating at120° C. for 2 minutes to form the coating film of the photosensitivepolyimide resin precursor. After irradiating the coating layer withultraviolet rays via a mask at a light exposure amount of 700 mJ/cm² andheating at 160° C. for 3 minutes, the exposed layer was subjected to adevelopment treatment to form negative-type images. The coating film wasfurther heated at 400° C. under a vacuum of 0.01 torr to form apatterned insulating layer (thickness 6 μm) composed of the polyimideresin to obtain a circuit substrate. The linear thermal expansioncoefficient of the polyimide resin measured as in Example 1 was 16.7ppm/° C.

A circuit-formed suspension substrate was obtained using the circuitsubstrate in the same manner as in Example 1.

In the circuit-formed suspension substrate thus obtained, since thepolyimide resin has a low coefficient of linear thermal expansion,cracking does not occur on the resin layer, delamination of the resinlayer from the substrate does not occur, and the circuit-formedsuspension substrate has a high reliability and does not cause warping,thereby being free from performance failure.

Example 3

In 19.72 kg of dimethylacetamide were dissolved 0.702 kg (6.5 mols) ofp-phenylenediamine, 1.535 kg (5.2 mols) of3,4,3′,4′-biphenyltetracarboxylic acid dianhyride, 0.577 kg (1.3 mols)of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane (the sum of the acidanhydrides: 6.5 mols), and the resulting solution was stirred at roomtemperature for 72 hours. The temperature of the solution was raised to75° C. When the viscosity of the solution reached 5,000 c.p., heatingwas stopped, and the solution was allowed to stand to cool it to roomtemperature.

By adding 0.9633 kg (2.78 mols) of niphedipin, 0.6422 kg (2.04 mols) ofan acetyl compound, and 0.161 kg (2.36 mols) of imidazole to thesolution, a solution of a photosensitive polyimide resin precursor wasprepared.

After coating the solution of the photosensitive polyimide resinprecursor on a long stainless steel (SUS 304) foil having a thickness of25 μm by a continuous coater, the coating layer was dried by heating at120° C. for 2 minutes to form the coating film of the photosensitivepolyimide resin precursor. After irradiating the coating layer withultraviolet rays via a mask at a light exposure amount of 700 mJ/cm² andheating at 160° C. for 3 minutes, the exposed layer was subjected to adevelopment treatment to form negative-type images. The coating film wasfurther heated at 400° C. under a vacuum of 0.01 torr to form apatterned insulating layer (thickness 6 μm) composed of the polyimideresin to obtain a circuit substrate. The linear thermal expansioncoefficient of the polyimide resin measured as in Example 1 was 17.3ppm/° C.

A circuit-formed suspension substrate was obtained using the circuitsubstrate in the same manner as in Example 1.

In the circuit-formed suspension substrate thus obtained, since thepolyimide resin has a low coefficient of linear thermal expansion,cracking does not occur on the resin layer, delamination of the resinlayer from the substrate does not occur, and the circuit-formedsuspension substrate has a high reliability and does not cause warping,thereby being free from performance failure.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A circuit substrate comprising a metal foilsubstrate and an insulating layer composed of a polyimide resin formedon the metal foil substrate, wherein the polyimide resin is one obtainedby the reaction of (A) p-phenylene diamine and (B) acid anhydrides of(a) 3,4,3′,4′-biphenyltetracarboxylic acid dianhydride and (b)2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane.
 2. A circuit substratecomprising a metal foil substrate and an insulating layer composed of apolyimide resin formed on the metal foil substrate, wherein thepolyimide resin is one obtained by reacting a polyamic acid obtained bythe reaction of (A) p-phenylene diamine and (B) acid anhydrides of (a)3,4,3′,4′-biphenyltetracarboxylic acid dianhydride and (b)2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane in the presence of aphotosensitive agent.
 3. The circuit substrate of claim 2, wherein thephotosensitive agent is represented by following formula (I);

wherein X¹, X², X³, and X⁴ each independently represent a hydrogen atom,a fluorine atom, a nitro group, a methoxy group, an amino group, adialkylamino group, a cyano group, or a fluorinated alkyl group; Y¹represents a cyano group or a group represented by a formula —COR³; Y²represents a cyano group or a group represented by a formula —COR⁴,wherein R³ and R⁴ each independently represent an alkyl or alkoxy grouphaving from 1 to 4 carbon atoms, an anilino group, a toluidino group, abenzyloxy group, an amino group, or a dialkylamino group; R¹, R², and R⁵each independently represent a hydrogen atom or an alkyl group havingfrom 1 to 3 carbon atoms; and at least one of the combinations of R¹ andR³, and R² and R⁴ may be combined to form a 5-membered ring, a6-membered ring, or a heterocyclic ring each containing a keto group. 4.The circuit substrate of claim 2, wherein the photosensitive agent is atleast one dihydropyridine derivative selected from the group consistingof 4-o-nitrophenyl-3,5-dimethoxycarbonyl,2,6-dimethyl-1,4-dihydropyridine,4-o-nitrophenyl-3,5-diacetyl-1,4-dihydropyridine, and4-o-nitrophenyl-3,5-dimethoxycarbonyl-2,6-dimethyl-1-methyl-4-hydropyridine.5. The circuit substrate of claim 1, wherein component (B) comprisesfrom 70 to 99 mol % of component (a) and from 30 to 1 mol % of component(b).
 6. The circuit substrate of claim 1, wherein an aminogroup-containing bifunctional polysiloxane is used in the range of 10mol % or less of the diamine component composed of p-phenylene diamineand the amino group-containing bifunctional polysiloxane, the aminogroup-containing bifunctional polysiloxane being represented by formula(II):

wherein R¹ represents an alkylene group having from 1 to 18 carbonatoms; R² represents an alkyl group having from 1 to 18 carbon atoms;and n represents an integer of from 1 to
 100. 7. A method of producingthe circuit-substrate as claimed in claim 1, which comprises forming acoating film composed of a photosensitive polyimide resin precursor on ametal foil substrate, light-exposing, heating after the light-exposure,developing, imidating the precursor by heating to form a circuitsubstrate having an insulating layer composed of a polyimide resin,wherein said photosensitive polyimide resin precursor is obtained bycompounding a polyamic acid obtained by the reaction of (A) p-phenylenediamine and (B) acid anhydrides of (a) 3,4,3′,4′-biphenyltetracarboxylicacid dianhydride and (b) 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanewith a photosensitive agent.
 8. The method of producing a circuitsubstrate of claim 7, wherein the photosensitive agent is represented byfollowing formula (I):

wherein X¹, X², X³, and X⁴ each independently represent a hydrogen atom,a fluorine atom, a nitro group, a methoxy group, an amino group, adialkylamino group, a cyano group, or a fluorinated alkyl group; Y¹represents a cyano group or a group represented by a formula —COR³; Y²represents a cyano group or a group represented by a formula —COR⁴,wherein R³ and R⁴ each independently represent an alkyl or alkoxy grouphaving from 1 to 4 carbon atoms, an anilino group, a toluidino group, abenzyloxy group, an amino group, or a dialkylamino group; R¹, R², and R³each independently represents a hydrogen atom or an alkyl group havingfrom 1 to 3 carbon atoms; and at least one of the combinations of R¹ andR³, and R² and R⁴ may be combined to form a 5-membered ring, a6-membered ring, or a heterocyclic ring each containing a keto group. 9.The method of producing a circuit substrate of claim 7, wherein thephotosensitive agent is at least one dihydropyridine derivative selectedfrom the group consisting of4-o-nitrophenyl-3,5-dimethoxycarbonyl-2,6-dimethyl-1,4-dihydropyridine,4-o-nitrophenyl-3,5-diacetyl-1,4-dihydropyridine, and4-o-nitrophenyl-3,5-dimethoxycarbonyl-2,6-dimethyl-1-methyl-4-hydropyridine.10. The method of producing a circuit substrate of claim 7, whereincomponent (B) comprises from 70 to 99 mol % of component (a) and from 30to 1 mol % of component (b).
 11. The method of producing a circuitsubstrate of claim 7, wherein an amino group-containing bifunctionalpolysiloxane is used in the range of 10 mol % or less of the diaminecomponent composed of p-phenylene diamine and the amino group-containingbifunctional polysiloxane, the amino group-containing bifunctionalpolysiloxane being represented by formula (II):

wherein R¹ represents an alkylene group having from 1 to 18 carbonatoms; R² represents an alkyl group having from 1 to 18 carbon atoms;and n represents an integer of from 1 to 100.